Introduction to Raster Image Processing

At the time of imaging, the OPI function replaces the low-res image with the proper high-res version of the image for halftone screening or large-format output. This is particularly beneficial when there are many images in a document. The OPI capability allows for smaller working documents and less processor-intensive video display.Mark Coudray

<P>However, OPI is losing favor due to the emergence of DCS and the DCS 2.0 file format. DCS was invented by Quark and is now almost universally accepted as a method of high-resolution substitution. To use DCS, you simply save the graphic file as DCS file format. DCS is a special type of EPS file and behaves in much the same way. It is very easy to place or import an image into a layout or illustration program using this format. For screen printers, DCS 2.0 works very well for documents that contain both process-color and spot-color elements.

This replacement of high-resolution images in working graphic documents has a natural and logical evolutionary trend that leads to variable data capabilities. As processors become faster and faster, the primary image is RIPed and cached, and specific or variable information is then merged with the primary image to create the final output. This is called variable-data printing, and it is an emerging technology that offers very interesting opportunities.

To make variable-data printing possible, the RIP must be able to work from image caches. This involves ripping several images and holding them in storage. Then, as jobs are printed, the specific data or variable data is merged with the stored images according to user-defined parameters. The result is specific output that matches the user needs.

An example might be signage that directs certain groups to certain activities. The basic poster would be the same, but the variable graphics relating to the groups and the activities would change with each sign.

Other image-related functions that are provided by RIPs include nesting and imposition. Nesting is the ability to fit or arrange multiple images across the smallest possible area of the output media for most efficient use of the media. An example for textile printers would be to nest three or four pocket logo designs across the width of the media instead placing only one image in the center.